Many attempts have been made to construct apparatus to accurately count passengers boarding and alighting buses and other transit vehicles by the method of: (1) determining the direction of movement of the person and (2) determining whether the entrance or exit operation was completed or aborted. Several methods have incorporated various types of switches into at least two of the consecutive step treads of the transit vehicle to sense the weight transmitted to them by passengers' feet. The sequences of closings and openings of these switches are analyzed by various forms of logic to determine both the direction of flow and the completion of the boarding or alighting operation. These switches are subjected to (1) millions of flexures in the course of use on a mass transit vehicle, and (2) environmental conditions such as moisture, snow and ice, dirt and dust, extreme fluctuations in temperature with door openings and changes in the seasons and weather, which all affect the life and the operation of the switches. Further problems result from wear of the tread surface which reduces the practicality of treadle switches even though the cost of these units is potentially low.
Other methods involve infrared or visible light sources and sensors with multiple light beams directed horizontally across an entrance or exit portal to detect both movement and completion of an event when these beams are interrupted in sequence by parts of the passenger's body. These methods suffer from the disadvantages of uncertainty of count produced as the result of interruptions by canes, umbrellas, coat sleeves, and other spurious objects which may also traverse the path and interrupt the beam in other than the desired sequences. These methods have the advantages that weather and atmospheric conditions are less detrimental than for treadle switches and the sensors usually exhibit longer life-times than treadle switches.
A third method uses ultrasonic sound waves transmitted either across the portal or out into the portal with detection by an ultrasonic receiver. Detection of parts of passengers' bodies are made by either interrupting the ultrasonic beam, keeping it from being sensed by an ultrasonic receiver, or by the reflection of the beam into a sensor by parts of the passenger's body. A modification of this method senses the reflection of a short pulse of ultrasound into a sensor sooner by an "observed" object that is closer to the ultrasound receiver than the opposite wall of the step well that normally reflects the pulse into the receiver in a given constant length of time when no object is present. The disadvantages of the method are that absorbing or deflecting materials such as clothing may either prohibit reflection of the beam or may reflect it in another direction so that it is not sensed by the sensor thus confounding the operation of the unit.
These and other methods may be found in a report: M74-86 Oct. 1974, prepared by the Mitre Corporation for the Urban Mass Transportation Administration (UMTA), and in a final report of the results of a study of these methods also prepared by the Mitre Corporation for UMTA. All of these methods are inaccurate as the result of the ambiguity in the meaning of the output of each sensor with respect to the output of the other sensors. The requirements placed on the logic to analyze a reasonable number of sensor outputs and convert them into meaningful outcome with respect to the direction and completion of the events is almost impossible because each output from a sensor may have more than one significance. For example, persons standing on the stairs will cause a constant output from one or more sensors and will most certainly prevent the detection of persons passing by them either on or off. Also persons moving back and forth such as a feeble person who is having trouble negotiating the stairs or a person asking a question of the driver will, in most cases, give an indication of either no count or multiple counts in both directions. Furthermore, persons crowding on or off the vehicle may confuse the logic by generating sensor outputs in a large variety of sequences and patterns which may have multiple meanings or no meaning at all. These problems in logic related to a reasonable device of reasonable complexity may be found in the Mitre Corporation report, MTR-7071, of the test of various methods of automatically counting boarding and alighting persons on transit vehicles.
It may be further pointed out that with the exception of treadle switches, which have their own problems including that of liability as the result of possible falls from tripping, all of the other counting methods described require the use of beams of radiation of some sort in order to detect the presence of passengers or objects. The required use of a beam is a drawback in itself in that failure or partial impairment of the source of one or more beams jeopardizes the ability of the device to detect and count objects. Blockages of one or more beams, such as with a hand, piece of clothing or object standing in front of them has the same effect. Effects that cause dimming or brightening of the beams, or ambient sources of radiation of the same type that penetrate into one or more of the sensors to the extent of masking, saturating, or in other words decreasing the sensors' ability to distinguish the directed beam from the ambient, impair the ability of the device to detect and count objects.